1. Fields of the Invention
The present invention relates to a measuring apparatus and a measuring method, especially to an apparatus and a method for measuring fluid viscosity.
2. Descriptions of Related Art
Viscosity is one of the important physical properties and is a measure of the resistance of a fluid. The more viscous the fluid is, the lesser its ease of movement. In our daily lives, there are phenomena related to the viscosity such as the taste of drinks, the degree of difficulty in painting, the writing fluency of a fountain pen. Among industrial technologies, there are also a lot of applications related to the viscosity such as a damping device, dip-feed lubrication, fuel oil transfer, and fuel oil atomization etc. The most common viscometers that measures fluid viscosity are rotary viscometers, glass capillary viscometers, and falling ball viscometer.
A rotary viscometer, as revealed in U.S. Pat. No. 5,287,732, and No. 6,240,770, includes an outer cylinder, an inner coaxial cylinder, and a test liquid in a measuring gap formed between the outer cylinder and the inner cylinder. By rotating the inner cylinder, tangential velocity of the fluid at the wall is changed. Both the torque obtained and the tangential velocity are substituted into Newton's stress-strain equation so as to get the viscosity of the test liquid. The rotary viscometer has advantages of short measurement time, and simple instrument operation. However, the gap between the outer cylinder and the inner cylinder is quite small. Once the fluid contains granules therein such as slurry or suspension, the rotary viscometer doesn't work and its testability is reduced. Moreover, the length of each cylinder is quite long for reducing measurement error. Thus the viscometer occupies space and costs a lot. Furthermore, the length of two cylinders, the gap, location, the shape of the bottom of the cylinder, friction, and residual liquid all have effects on the measurement precision and cause increased measurement error. While measuring viscosity of different fluids, the viscometer needs to be disassembled and cleaned so as to prevent unexpected errors caused by residual fluid. Therefore the management and maintenance of the rotary viscometer are more difficult.
As to the glass capillary viscometer, it includes a U-shaped glass tube with a test liquid therein and two glass bulbs, also known as Ostwald viscometer. One arm of the U-shaped tube is a capillary and the other arm is a normal tube respectively connected to a glass bulb. One glass bulb is lower down than the other one. Due to gravity, the liquid flows in the capillary. The flow rate of the liquid in the capillary is obtained indirectly by control of the level of the liquid in two glass bulbs. By substituting the known flow rate into an equation for viscosity versus flow rate, the viscosity of the test liquid is obtained. Compared with rotary viscometer, the advantage of the glass capillary viscometer is with higher testability, less space and lower cost. However, in order to reduce measurement error, the two glass bulbs should be with larger volume. Moreover, up and down movements of the fluid level take a long time so that the measurement time is extended. Furthermore, the movements of the liquid level in the two glass bulbs cause hydrostatic pressure changes at the outlet end and this influence the measurement precision and further the measurement error is increased. In addition, the capillary diameter is quite small and difficult to be cleaned properly. Thus the maintenance is getting difficult. The liquid level moves freely in the two glass bulbs so that the operation for control of level movement is quite complicated. Besides, once the liquid has high viscosity or granules whose diameter is closed to the capillary diameter, the glass capillary viscometer is not suitable. The testability of the glass capillary viscometer is not high. In order to reduce measurement errors, the volume of each glass bulb is quite large so that the space required is increased. And the capillary is an integrated tube produced with high technical cost. Thus the high cost is still an issue.
Refer to Taiwanese Pat. Pub. No. 200912277, a conventional falling ball viscometer consists of a vertical tube and a ball. A test liquid is in the vertical tube. The ball is allowed to descend through the liquid in the vertical tube due to gravity. A velocity of the falling ball is learned and is substituted into an equation for viscosity versus velocity. Compared with the glass capillary viscometer, its advantages are short measurement time, low instrument cost, and easy operation. However, it's difficult to observe and measure if the test liquid is not clean and transparent. The testability is still not improved. Moreover, the ball size compared with the vertical tube, the surface deterioration and abrasion all have affects on the measurement precision. Thus the measurement error is large. The ball is a consumable and this causes difficulties in maintenance.
In order to overcome above disadvantages, there is a need to provide an apparatus and a method for measuring viscosity that not only improves testability but also reduces measurement time, space required, instrument cost, measurement error, difficulty in maintenance and operation complexity for solving the problems.